1. Field of the Invention
This invention relates to workpiece supports for supporting a workpiece and, in particular, but not exclusively, to a workpiece support for supporting a workpiece during a machining or routing operation.
2. Discussion of Prior Art
There are many applications where it is required to support one of a number of workpieces of different profiles during a machining or routing operation. For example, the leading edge skin of a typical commercial aircraft wing is made up of about a dozen sections of the same general shape but of different detailed profile. In an existing proposal, a sheet element is stretch-formed over a stretch-form of the required three-dimensional profile. After this initial forming the component is routed out of the stretch-formed skin. The stretch-forming and routing operations include six different stages as set out in FIGS. 1(a) to (f) of the accompanying drawings. Initially the margins 12 of the skin 10 are clamped in the jaws of a stretch form machine and then stretch-formed around a profile to -give the required shape. At this stage, although the overall profile of the skin is formed, the component itself still needs to be cut out of the stretch-formed skin using e.g. a CNC routing machine, and it is essential that the skin be accurately located prior to the routing operation. Thus in stage 2 the stretch-formed surplus is removed and pilot holes 14 are drilled in the opposite edges of the workpiece 10 which are enlarged to form location or component clamping holes. In stages 3 and 4 the trimmed workpiece 10 is applied to a routing fixture 16 which has a series of manually operated toggle clamps 18 which engage in the component clamping holes 14 and pull the workpiece accurately into alignment on the routing fixture 16. The profile is then routed leaving tags 20. In stage 5 the workpiece 10 is removed from the routing fixture 16, and in stage 6 the component is snipped out of the workpiece 10, detagged and deburred.
There are several disadvantages with this process. It requires a separate dedicated fixture and manually operated clamps for each of the different profile sections of the aircraft wing. In a typical example there may be thirteen different profiles for each wing, making a total of twenty-six hand and opposite routing fixtures. This constitutes a considerable capital outlay for each of the fixtures and a considerable storage requirement.
After stretch-forming, two tooling holes are drilled, the clamping margin 12 or xe2x80x9cflashingxe2x80x9d has to be removed before final machining. In order to assist in clamping and accurate location of the workpiece, several component clamping holes need to be drilled in each of the opposite edges of the workpiece. Because the workpiece 10 is held on the routing fixture 16 by a non-selective attachment (that is the toggle clamps 18) it is necessary to leave tags around the periphery of the component to keep it in the correct orientation and maintain clamping forces on the component. The tags then have to be removed by hand machining which adds to the process steps.
EP-A-0179957 discloses an arrangement for holding contoured workpieces which comprises a lattice arrangement of spring strips each having a series of suction cups standing proud of the strip for holding the workpiece by suction. In this arrangement the workpiece is supported at a distance from the lattice strips which means that it is not supported apart from at the suction cups. This support only at spaced point regions may provide inadequate support during machining, leading to vibration, and machining inaccuracies. Also, the lattice structure is essentially permanent, there being no apparent provision for readily removing individual members thereof. Thus changes in profile are achieved by flexing the lattice to a different configuration by rams at spaced points. This means that the configuration may only be an approximation to the required shape.
Accordingly, in one aspect, this invention provides a workpiece support for supporting a workpiece of a given profile, and being reconfigurable in use to support workpieces of a different profile, said workpiece support including:
a base means having a plurality of anchorage means at spaced intervals;
a plurality of station frame elements for being removably secured to said anchorage means to provide in use support for a given workpiece over an extended area at spaced stations, and
attachment means associated with each of said frame elements and being operable in use locally to secure the workpiece to said frame element.
In this arrangement, the station frame elements provide support over an extended area, typically a line or narrow strip contact of continuous or interrupted form, extending across a portion of the workpiece. This provides a secure location for the workpiece. In addition, the frame elements are removable. By making up the workpiece support from a series of spaced removable station frame elements, rather than being in the form of a single integral routing fixture, it is possible to provide support for a wide range of different profiled workpieces by having a library of differently profiled station frame elements from which the required station frame elements for a particular workpiece profile may be selected and anchored to the base. In addition, where the workpieces are handed, or symmetrical, for example the leading edge skin sections of an aircraft, the left and right hand body panels of a car etc., the workpiece support may be reconfigured between the symmetrical pairs simply by releasing each of the frame elements in turn, rotating them through 180xc2x0 and re-anchoring them to the base means. Furthermore, the library of frame elements may be stowed is compactly in a stacked or side-by-side arrangement, thereby considerably reducing the storage requirements compared to the existing fixtures. This leads to a consequent reduction in terms of set up time and lead time, and a reduction in tooling budget.
Although the attachment means for securing the workpiece to the workpiece support may take many forms, it is preferred for it to comprise suction means. The suction means advantageously comprises a plurality of suction cups of resiliently deformable material spaced on the frame elements to face the underside of the workpiece in use and to draw the workpiece down onto the frame element.
An important advantage of the use of suction cups, or other localised suction application, is that the suction may be both localised and independent of the suction at other positions. Thus, given suitable spacing of the frame elements and the suction cups it is possible to rout around the entire periphery of a component, without having to leave tags as previously. In other words, since the component and the scrap are individually and independently clamped by the suction cups there will be little, if any, displacement of the component relative to the scrap when the periphery is routed. This provides an important advantage because it means that the manual snipping and de-tagging operations are no longer necessary.
Where the workpiece has a convex or concave profile, the attachment means are preferably disposed one at or adjacent the peak or trough of the profile and one or more on the flanks of the frame elements to either side thereof.
Preferably, at least one of the frame elements includes location means for co-operation in use with corresponding location means on the workpiece to locate the workpiece relative to said workpiece support. Conveniently, the two outermost frame elements may each include such location means. The location means may typically comprise a pin or spigot on the frame elements which engages a location hole on the workpiece.
Using the frame element to provide the location means which locate the workpiece obviates requirements in previous proposals to trim the flashing following stretch-forming, and to drill a series of clamping holes in opposite sides of the stretch-formed item. Again this reduces the number of process operations and thus cost.
Preferably, said anchorage means includes clamp means for releasably clamping said frame elements to the base. The clamp means preferably allow the frame elements to be clamped thereto in either of two orientations spaced by 180xc2x0. The clamp means may be hydraulically operated.
Preferably, said anchorage means are spaced at equal intervals.
The invention also extends to a machine tool including a workpiece support as set out above.
In another aspect, this invention provides a method of providing support for one of a number of workpieces of different profiles, which includes:
providing a base means having a plurality of anchorage means at spaced stations;
providing a plurality of frame elements corresponding to the profiles of said workpieces at said spaced locations;
selecting for a given workpiece the frame elements corresponding to the profiles at said stations;
removably securing said frame elements to said base means to define a reconfigurable workpiece support; and
removably securing said workpiece to said reconfigurable workpiece support.
Where at least two of said workpieces have profiles which form a generally symmetrical pair, the same frame elements are preferably used for each of the pair, with the order and/or orientation of the frame elements being reversed.
Whilst the invention has been described above, it extends to any inventive combination of the features set out above or in the following description.